After some updates including nvidia 352.79 and some more update which had
destroyes the libgl.so link to nvidia(!) module.  I see a black screen on
starting lightdm ... and with debugging, which means starting e.g.

   X :1 vt8

and running

   DISPLAY=:1 xrandr --setprovideroutputsource modesetting NVIDIA-0
   XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":1"
      after 17 requests (17 known processed) with 0 events remaining.

and the last famous lines of the X server looks like

   (EE) 
   (EE) Backtrace:
   (EE) 0: X (xorg_backtrace+0x48) [0x58b128]
   (EE) 1: X (0x400000+0x18f339) [0x58f339]
   (EE) 2: /lib64/libc.so.6 (0x7f037131f000+0x35200) [0x7f0371354200]
   (EE) 3: /usr/lib64/xorg/modules/drivers/nvidia_drv.so
(0x7f036b256000+0xc1604) [0x7f036b317604]
   (EE) 4: /usr/lib64/xorg/modules/libwfb.so (wfbSolid+0x369) [0x7f036ac1c4a9]
   (EE) 5: /usr/lib64/xorg/modules/libwfb.so (wfbFill+0x50a) [0x7f036ac11bda]
   (EE) 6: /usr/lib64/xorg/modules/libwfb.so (wfbPolyFillRect+0x1a0)
[0x7f036ac12710]
   (EE) 7: /usr/lib64/xorg/modules/drivers/nvidia_drv.so
(0x7f036b256000+0x5ced82) [0x7f036b824d82]
   (EE) 8: X (0x400000+0x1184a6) [0x5184a6]
   (EE) 9: X (miPaintWindow+0x1ce) [0x56e3ae]
   (EE) 10: X (miWindowExposures+0x59) [0x56eaf9]
   (EE) 11: X (0x400000+0x9341a) [0x49341a]
   (EE) 12: X (miHandleValidateExposures+0x66) [0x583026]
   (EE) 13: X (SetRootClip+0x1d0) [0x46c690]
   (EE) 14: X (0x400000+0xbae89) [0x4bae89]
   (EE) 15: X (ProcRRSetProviderOutputSource+0xd1) [0x4fca81]
   (EE) 16: X (0x400000+0x3d1de) [0x43d1de]
   (EE) 17: X (0x400000+0x40fbb) [0x440fbb]
   (EE) 18: /lib64/libc.so.6 (__libc_start_main+0xf5) [0x7f0371340b05]
   (EE) 19: X (0x400000+0x2c5de) [0x42c5de]
   (EE) 
   (EE) Segmentation fault at address 0x0
   (EE) 
   Fatal server error:
   (EE) Caught signal 11 (Segmentation fault). Server aborting

this looks like a reace condition as the xrandr file in /etc/X11/xdm/Xsetup
seems to do something but does not get the nvidia driver to do any graphical
output.